1. Field of the Invention.
This invention pertains generally to so-called sine or tangent bar devices utilized for relating the angle of inclination of a measuring plane to the linear displacement of an end of the measuring plane, and more particularly to sine bar type devices suitable for use in optics.
2. Description of the Prior Art.
It is commonly desired in precision machining operations to precisely determine the angle of a machine part, and as a result a number of devices have been developed to allow the setting of such angles to a higher degree of precision than is attainable with common angle measuring devices such as protractors and the like. These devices, commonly known as sine bars, typically consist of a precision machined planar member which rests at one end upon a machined flat member, and is raised at the other end a precisely determined linear distance. The sine of the angle of inclination is directly related to the linear distance that the one end of the planar member is displaced. The displacement of the planar member, or sine bar, is typically accomplished by supporting the end of the planar member on precisely machined gage blocks or by a calibrated precision drive such as a micrometer.
Sine bar type devices also find application in optics wherein it may be desired to precisely set the angle of a mirror, lens, or diffraction grating relative to the direction of incoming light. A relatively recent development requiring precision angle setting is the use of "Echelle" gratings as optical filters in tunable narrow bandwidth lasers. The Echelle grating has the property of reflecting directly back towards a light source only a narrow band of light frequencies within the frequency of the light that is directed upon the grating. The midfrequency of the light that is reflected directly back to the light source is dependent upon the angle of incidence of the light which is directed upon the grating. Precise setting of the angle of the Echelle grating allows precisely selected frequencies to be directed back to the source of laser light, thereby allowing the laser to be tuned to the desired frequency by adjusting the angle of the grating. See, e.g., J. E. Lawler, W. A. Fitzsimmons, and L. W. Anderson, "Narrow Bandwidth Dye Laser Suitable for Pumping by a Short Pulse Duration N.sub.2 Laser," Applied Optics, Vol. 15, p. 1083 et seq., April 1976. The precision setting of the angle of the Echelle grating is preferably accomplished by the use of a precision sine bar type device.
The typical adjustable sine bar is fixed at one end of the planar member and is driven upwardly by a spindle of a micrometer drive at the other end. Such a device is obviously limited in the angles to which the micrometer drive can displace the end of the sine plate. A straightforward response to this problem has been to mount the micrometer itself for sliding movement so that the micrometer can be moved inwardly toward the pivot point of the planar member as the planar member is moved upwardly toward very large angles. However, this requires precision adjustments both of the displacement of the spindle of the micrometer and the displacement of the micrometer inwardly toward the pivot point. These problems become especially acute in the use of sine bars for mounting Echelle gratings since it is desirable to be able to operate the Echelle grating over relatively large changes, in angles of incidence, from relatively small angles of incidence up to relatively large angles of incidence in the range of 70.degree. to 80.degree., while maintaining a high degree of precision.